Timing Functions of the Cerebellum

Timing Functions of The Cerebellum Richard B. Ivry and Steven W. Keele University of Oregon and Good Samaritan Hospital, Portland, Oregon ' Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/1/2/136/1755571/jocn.1989.1.2.136.pdf by guest on 18 May 2021 Introduction must address the computational problems which need to be solved (Marr, 1982). The concept of the motor program has proved useful to With this in mind, the work in our laboratory over the researchers attempting to explain how voluntary move- past few years has focused on delineating the basic com- ments are controlled. As developed by Henry and Rogers ponents of coordination. The underlying premise is that (1960) and extended by Keele (1968) among others, the the term "coordination" provides only a general charac- motor program is an abstract representation of an intended terization of skill. More insightful understanding can be movement, containing not only the goal of the action and obtained by decomposing this general term into a number the existing environmental conditions, but also the pos- of separable operations. In this sense, skilled behavior sible means by which the movement could be achieved. would be explained in terms of the successful execution The motor program is analogous to the role of software of these independent procedures. For instance, compu- used by computers-flexible as a function of the input, but tational modules might include procedures to control the constrained within the limitations of the hardware. The sequence of actions and their locations, the selection of program is the most general description of the capabilities the appropriate muscles to implement each action, and of the system. the specification of the force and time parameters for each A logical extension of the computer metaphor is to of the selected muscles (but see Stein, 1982; for a consider the internal procedures or operations which may discussion of higher order control variables see Flash & constitute the motor progmm. This would also be a Hogan, 1985; Soeehting & Lacquaniti, 1981). prerequisite for developing a computational model of Our initial efforts to determine the validity of candi- motor control. The highest level description of any model date components of coordination used a correlational ap- 136 Iournal of Cognitiw Neuroscience Volume I, Number 2 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1989.1.2.136 by guest on 25 September 2021 proach. In our first study on timing control (Keele, ago (Braitenberg, 1967, 1965; Braitenberg & Onesto, Pokorny, Corcos, & Ivry, 1985b), we found that subjects 1962). Braitenberg argued that information concerning who were good at maintaining an arbitrary rhythm with the common source (i.e., mossy fiber input) generating a one effector such as the hand also tended to be good at the specific parallel fiber pulse would be lost unless the output same task when using a different effector such as the foot. system accounts for the different time delays introduced as More surprising, a significant correlation wak found be- the parallel fiber traverses an orthogonal array of Purkinje tween subjects’ ability in timing production tasks and tests cells. For this operation, Braitenberg (1967) proposed a of timing perception when the durations of time are com- scheme based on delay lines which vary as a function of parable across the two domains. Subsequent studies the distance between the input and output signal. Since (Keele, Ivry, & Pokorny, 1987) showed the ability to the conduction velocity within these fibers is a relatively Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/1/2/136/1755571/jocn.1989.1.2.136.pdf by guest on 18 May 2021 control time to be largely independent of the ability to slow 0.5 dsec.(Braitenberg & Atwood, 19581, a 100 control force arguing for the separability of processes mm. chain of fibers extending across the cerebellum could controlling force and time. These results were interpreted provide a delay signal as long as 200 ms. Shorter chains as providing evidence for a common timekeeping mecha- could presumably be activated for shorter temporal inter- nism which is used in both production and perception vals. functions which involve time-related decisions. Thus, the Despite the elegance of the theory, Braitenberg has componential approach provides a more abstract defini- since come to view the basic conception of simple delay tion of the organisms’capabilities. An operation may not lines as inadequate. Neurophysiological data indicated be best thought of as part of the motor or perceptual that somatotopic representations within the cerebellar system, but rather as an independent entity which is cortex span relatively short distances (Oscarsson, 1980; employed whenever its specific computation is needed. Robertson, 1985). The maximum delay signal which could Neuropsychological research provides a second way be acheived in this distance is too small to be meaningful to investigate the validity of these hypothesized proce- in motor coordination (Fahle & Braitenberg, 1984). dures. The correlational work has yielded model tasks Nonetheless, Braitenberg has not entirely dismissed which can assess the functioning of separable compo- the role of timing in his recent conjectures on cerebellar nents. Different patient groups can then be tested in an function. A recent model (Fahle & Braitenberg, 1984) effort to show dissociations between the patients’ per- postulates that the cerebellum establishs synchrony be- formance of these tasks as a function of the location of their tween the dynamic events associated with multi-joint neurological lesion. This paper will present results of our movements and the subsequent mechanical conse- neuropsychological work on timing. quences. In the same spirit, Pellionisz and Llinas (1982) Previous research in our laboratory (Wing, Keele, & have argued that the cerebellum can be viewed as a Margolin, 1984) had revealed a timing deficit, at least on neuronal device for jointly mapping space and time onto the production task, in a patient with Parkinson’s Disease. a common dimensional space. In both of these models, The primary neurological damage in Parkinson’s Disease time is part of the computational process since the cere- is presumed to be in the dopamine pathways of the basal bellum needs to anticipate the multiple joint positions ganglia. In another single subject study (Keele, Manch- which will be acheived during the course of a movement. ester, & Rafal, 1985a), a patient with damage to the While the preceeding arguments were based on ana- cerebellum was also found to have difficultyin producing tomical observations, many clinical and experimental regular timed intervals. Taken together, the two case results can also be interpreted as supportive of the hy- studies would appear to implicate both subcortical struc- pothesis that the cerebellum may function as a timing tures in timing functions and thus promote the argument device. The pioneering work of Holmes (summarized in that timing may involve some sort of pathway which Holmes, 1939; see also Dichgans & Diener, 1984) identi- passes through both regions. Alternatively, only one of fied two of the more common symptoms of cerebellar dys- these regions, or some other unexplored region may be function: dysmetria and dysdiadochokinesia. Following critical for timing functions. If this were the case, the lesions of the cerebellar hemispheres or the deep cerebel- effects observed in the case reports may be due to indirect, lar nuclei, particularly the lateral zones and the dentate modulatory effects that the basal ganglia and/or cerebel- nucleus, the patient’smovements are usually hypermetric. lum exert on the timing mechanism. The perception of Dysdiadochokinesia, the inability to rapidly alternate time task offers one way to test whether a particular neural between a pair of movements involving antagonist structure plays a primary role in timing since the response muscles, is generally seen in these same patients (Eccles, requirements for this task are minimal and independent of 1977). Both of these cerebellar signs have been inter- those required in the production task. preted as being the result of a breakdown in the patient’s Unlike the basal ganglia, there are a number of con- ability to time the onset and offset of antagonist muscles. verging lines of evidence which suggest that one of the For instance, hypermetric movements may overshoot the functions of the cerebellum involves timing control. The target because the agonist activity is not properly termi- most explicit statement of the cerebellum as an internal nated. clock was put forward by Braitenberg over twenty years Evidence from researchers using electromyography Imy and Keele 137 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1989.1.2.136 by guest on 25 September 2021 (EMG) supports this hypothesis (Hallett, Shahani, & (McCormick & Thompson, 1984). Note that the condi- Young, 1975; Marsden, Merton, Morton, Hallett, Adam, & tioned response (CR) is linked to the onset of the aversive Rushton, 1977). Hallett et al., (1975) examined the EMG stimulus (US) which follows the CR rather than to the records of patients who had incurred various forms of conditioned stimulus (CS) which preceeds the CR. The cerebellar lesions. They found that cerebellar patients did anticipatory nature of the CR strongly suggests explicit not tend to show any deficit in making smooth, slow timing. Disruption of the timing of the CR following movements which only required the continuous activation lesions of the cerebellar hemisphere further supports the of the agonist muscle. However, when the movements of hypothesis that the cerebellum plays a primary role in the same amplitude were made ballistically, almost all of timing. the patients showed EMG abnormalities. In general, the It should be made clear, though, that the abnormali- Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/1/2/136/1755571/jocn.1989.1.2.136.pdf by guest on 18 May 2021 EMGs associated with rapid movements appeared normal ties in the EMG profile and the conditioning results can in terms of the initial onset of the agonist burst.

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